Wireless and data communication system use continues to increase, creating an ever-growing need to exploit all available frequency bands. Starting at 900 MHz GSM bands, 1800 MHz DCS bands, and 2.4 GHz/5 GHz LAN bands, the quest for additional band width permitting higher speed data transfer is on going. Ultimately, it may be necessary to have a front end that will be automatically reconfigured, in response to the demand for the requested service or the overall traffic congestion. Ideally, the reconfiguration would occur automatically, without any input from the user.
One of the challenges to achieving an automatically reconfigurable front end is the need for a wide range of characteristics over a wide frequency range within a global circuit configuration. Attempts have been initiated in dual frequency, but often assuming that one frequency is double the other. See S. Wu, B. Razavi, “A 900 MHz/1.8 GHz CMOS Receiver for Dual Band Applications,” IEEE ISSCC Symposium, San Francisco, 5–7 Feb. 1998, p. 124–125. As such, the previously proposed configurations would not provide the desired unconditional multimode front end.
In addition a 5 GHz-band multifunctional BiCMOS transceiver chip for gaussian minimum-shift keying (GMSK) modulation wireless systems has previously been proposed. See M. Madihian, T. Drenski, L. Desclos, H. Yoshida, H. Hirabayashi, T. Yamazaki, “A 5 GHz Band Multifunctional BiCMOS Transceiver Chip for GMSK Modulation Wireless Systems,” IEEE Journal of Solid State Circuits, vol. 34, No. 1, January 1999, pages 25–32. One shortcoming of the proposed transceiver is that it is limited to a specific frequency band. In addition, the disclosed transceiver lacks versatility because it doesn't include means for providing different matching for the input and output of its constituent amplifiers for different frequencies.
Active inductors are one possibility for providing adaptive matching. In the field of circuit design, several types of inductors are used. Generally, inductors can be classified as passive or active. FIG. 1 shows an example of a passive inductor. A first metal layer ML1 having a width w is represented by the dark region and a second metal layer ML2 having a width is depicted by the white region. The two widths w and s and the four lengths L1, L2, L3 and L4 determine the value of the inductance and the quality factor, thereby fixing the values of these two physical parameters. One shortcoming of passive inductors is that they are often limited in quality factor (Q) by the metal layer thickness, the relative permitivity of the oxide, and losses through the substrate. Typical inductance and quality factor values are several nano Henry and a Q of 5, respectively. Suppression of losses and high inductance values can only be achieved in these passive inductors by resort to a more complex process. For example, one possibility is to remove the Silicon under the passive inductor. However, even if the passive inductor's quality factor and inductance increase, these values are fixed based on the physical characteristics of the passive inductor.
In order to provide a versatile means for adjusting inductance, several researchers have proposed the concept of active inductors. Several examples of such artificial active inductors have been proposed in the literature, and one of the most common was proposed by Hara. See S. Hara, T. Tokumitsu, M. Aikawa, “Lossless Broadband Monolithic Microwave Active Inductors”, IEEE Trans on MTT, vol 37, n 12, December 1989. These active inductors are essentially formed of transistors, capacitors and resistors. Under several assumptions based on the transistor model, the equivalent circuit behaves like an inductor.
The structure proposed by Hara is depicted in FIG. 2 and includes three transistors T1, T2, T3, which are connected as follows.
As shown in FIG. 2, an emitter of the transistor T1 is connected to an output port Vout and connected to a collector of the transistor T2. A base of the transistor T2 is connected to a voltage supply VS 1, as well as between two capacitors C1 and C2 which are connected together. An emitter of the transistor T2 is connected to a collector of the transistor T3.
An end of capacitor C2, which is not connected to the base of transistor T2, is connected to a base of T1 and to a voltage supply VS2. An end of capacitor C1, which is not connected to the transistor T2, is connected to an emitter of T3, to a voltage supply VS3, and to a load L1 which consists of a capacitor and resistor.
A collector of the transistor T1 is connected to a voltage supply VS4 and a capacitor C11. The other end of the capacitor C11 is connected to the base of T3 and to a voltage supply VS5. In the circuit illustrated in FIG. 2, the capacitor C11 is used as a DC buffer between the two transistors T1 and T3.
One of the drawbacks of the structure illustrated in FIG. 2 is that it uses a Direct Current Cut capacitor C11 in the loop of the structure. From an Alternate Current perspective, the losses occurring in the loop are critical for good performance. Therefore, in silicon based devices, the losses occurring in the capacitors, especially in the case of MIM capacitors, are so high that they prohibit the formation of an active inductor for the structure illustrated in FIG. 2. Thus, in order to use the active inductor illustrated in FIG. 2, one must minimize the capacitor C11 in order to minimize leakage into the substrate and the corresponding destruction of the inductive effect. On the other hand, if the capacitor C11 is not large enough, it will act as a high pass filter. As a result, the inductive effect can be achieved using the circuit illustrated in FIG. 2 only at high frequencies, typically around 10 GHz. However, the current state of bipolar technology using Silicon is not capable of attaining such high frequencies. Because the gain of the equivalent inductive element is dependent upon the gain and the parasitic elements of the constituent bipolar transistors, it is almost impossible to realize active inductors on Silicon substrate for intermediate and low frequencies (1 GHz to 6 GHz).